1. Field of the Invention
The present invention concerns a mixer circuit of the type having a first input, a second input, a frequency mixer and an output.
2. Description of the Prior Art
A mixer circuit of the above type is known from DE 102 19 749 A1. In this mixer circuit a first input signal can be fed to the frequency mixer via the first input, and a second input signal via the second input. The first input signal exhibits a first input frequency, the second input signal exhibits a second input frequency. The frequency mixer has an amplifier element by means of which it mixes the input signals with one another into a mix signal. The mix signal has a first signal component and a second signal component, the first signal portion exhibiting a component frequency and the second signal portion exhibiting a second component frequency. The first component frequency is equal to the sum of the input frequencies and the second component frequency is equal to the difference of the input frequencies. An output signal that is one of the signal component of the mix signal can be tapped via the output. The second input frequency is significantly higher than the first input frequency.
A mixer circuit with a first input, a second input, a frequency mixer and an output is likewise known from the textbook “Electronic Engineers Handbook” by Donald G. Fink and Donald Christiansen, McGraw-Hill, 1982, page 14-62. Here as well input signals that exhibit corresponding input frequencies can again be fed to the inputs. Furthermore, here the frequency mixer also mixes the input signals into a mix signal with two signal components, one signal component having frequency that is the sum of the input frequencies and the other signal component having a frequency that is the difference of the input frequencies. An output signal that has at least one of the signal components of the mix signal can again be tapped via the output. The frequency mixer has at least two amplifier elements by means of which it mixes the input signals with one another. It is fashioned as a balanced frequency mixer in which at least the second input and the output are decoupled from one another.
Further mixer circuits are described in DE 41 26 537 A1 and the textbook “The Art of Electronics”, Paul Horowitz and Winfield Hill, 2nd edition, Cambridge University Press 1989.
Varactor diodes can be operated as components known as parametric up-mixers (forward mixers). If a high mix amplification of the first input signal is desired in a parametric up-mixer, the second input frequency must be higher by a large factor than the first input frequency. The relative interval of the two component frequencies from the second input frequency is therefore rather small.
If an unbalanced frequency mixer is used as a frequency mixer, selective filters that exhibit a high performance (quality) must be used at the second input and at the output.